GB2415019A

GB2415019A - A supporting arrangement for a roller bearing

Info

Publication number: GB2415019A
Application number: GB0412666A
Authority: GB
Inventors: Barrie Dudley Brewster
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2004-06-07
Filing date: 2004-06-07
Publication date: 2005-12-14
Also published as: GB0412666D0

Abstract

A bearing arrangement for supporting a shaft 22 during rotation comprises a stationary housing 25, a rolling bearing 21 and a bearing support mechanism 23. The bearing support mechanism 23 includes a plurality of axially aligned members 24 positioned about the rolling bearing 21, a first end of each member 24 being attached to an outer race of the rolling bearing 21 and a second end of each member 24 being attached to the housing 25. Preferably the axial support members 24 are flexible rods, the ends of which are attached to the outer race and housing 25 by first and second locator rings 26, 27 respectively. The bearing support mechanism 23 is preferably made from a thermally conductive material such as beryllium copper or phosphor bronze. The arrangement is for use in a rotating machine such as a vacuum pump.

Description

1 2415019 Bearing arrangement The present invention relates to the field

of bearings in rotating machines, in particular to the supporVmounting of such bearings.

Where one or more rolling element bearings are incorporated into rotating machinery it is desirable to minimise transmission of vibration from the bearings to a housing of the machine. In order to achieve this, the bearings are typically mounted, in relation to the housing, using compliant components that will minimise such transmission. Meanwhile it is also necessary to preserve the relative axial location of the shaft in order to maintain the necessary clearances between a rotor and a stator of the pumping mechanism.

Figure 1 illustrates a rolling element bearing 11 mounted on a shaft 12 within a conventional pumping system, for example, in a molecular vacuum pump.

Such a system typically supports the bearing 11 within a housing unit 15 using both an axial support ring 13 and a radial support ring 14. These support rings are typically made of an elastomeric material. The total radial stiffness of the support arrangement is the sum of the stiffness of the radial support ring 14, and a further stiffness relating to shear deformation of the axial support ring 13. Alternatively, in the prior art, the bearing may be supported in the axial direction by a hard metal shoulder, forming part of the bearing housing.

In some circumstances, it is necessary to subject the bearing 11 to large axial loads. This may be due to a preload force being necessary to ensure proper operation of the bearing. Alternatively it may be due to a load caused by gas forces within the pumping mechanism, or due to a load being imposed by another (e.g. magnetic) bearing. High loads are likely to cause large deformation of the axial support ring, leading to poor accuracy in the location of the rotor. This, in turn, may prevent the maintenance of small running clearances in the pumping mechanism leading to a reduction in pumping efficiency or even potential seizure of the pumping components.

2 M04B108/KCM The stiffness characteristics of most elastomeric materials are highly non- linear. These effects are particularly noticeable when the elastomers are exposed to high levels of loading. Extreme loading may, therefore, lead to a larger contribution to the radial stiffness of the system from the axial support ring 13. This, in turn, leads to large transmission of radial vibration from the bearing 11 to the housing 15. In addition, most elastomers have very poor thermal conductivity, such properties leading to high bearing and rotor temperatures.

Furthermore, where a hard metal shoulder is used to support the bearing in the axial direction, frictional forces prevent free and unrestricted motion in the radial direction. Indeed, any radial oscillation of the bearing may give rise to fretting and wear, leading to the generation of wear debris which could enter the bearing and cause damage.

It is an aim of the present invention to overcome or at least minimise the aforementioned problems.

According to the present invention there is provided, a bearing arrangement for supporting a shaft during rotation, the bearing arrangement comprising: a rolling bearing supporting a portion of the shaft, and a bearing support mechanism comprising a plurality of axially aligned members positioned about the rolling bearing, a first end of each axially aligned member being attached to an outer race of the rolling bearing, a second end of each axially aligned member being attached to a non-rotating housing component By using an array of axially aligned members within the bearing support mechanism, a mechanism with high axial stiffness but a relatively low radial stiffness is provided that is able to resist axial movement of the rolling bearing and whilst allowing radial movement of the rolling bearing.

The axially aligned members may be rods. These rods may be flexible. Each of the members may be bonded or otherwise attached at one end to a locator 3 M04B1 08/KCM ring and the other end of each rod may be bonded or otherwise attached to a second locator ring. Alternatively the members may be integral with one or both locator rings. The first locator ring may be attached to the outer race of the rolling bearing and the second locator ring may be attached to the non- rotating housing component.

The bearing support mechanism may be made from a material which has a high thermal conductivity property to enhance the heat transfer from the rolling bearing to the housing.

A support ring made, for example, from an elastomeric material may be positioned about the bearing support mechanism and used in conjunction therewith.

A rotating machine having a bearing arrangement as aforementioned may be provided. More particularly, a vacuum pump may be provided comprising a pumping arrangement driven by a shaft rotated by a motor, with a bearing arrangement as aforementioned positioned about the shaft of the vacuum pump.

The invention is described below in greater detail, by way of example only, with reference to the accompanying drawings, in which Figure 1 is a schematic cross section of a conventional bearing arrangement; Figure 2 is a schematic cross section of one embodiment of the present invention; and Figure 3 is a schematic cross section of a second embodiment of the present invention.

Figure 2 shows a rolling element bearing 21 mounted on a shaft 22 of a rotating machine. The conventional support rings 13, 14 used in Figure 1 to position the bearing 1 1 within the housing 15 are replaced in this embodiment 4 M04B108/KCM with a bearing support arrangement 23. Bearing support arrangement 23 is positioned between bearing 21 and housing unit 25 to minimise transmission of vibration from the bearing 21 to the housing 25, whilst maintaining the axial position of the bearing 21 relative to the housing 25.

Bearing support arrangement 23 comprises an array of rods 24 positioned around the circumference of the bearing and between a first locator ring 26 and a second locator ring 27. Rods 24 are provided to confer a good resistance to axial displacement of the shaft 22 relative to the housing 25 to enable accuracy of the clearances between pumping components. The configuration of the bearing support arrangement 23 is such that, although it has a high resistance to axial displacement, radial and torsional resistance is significantly lower which enables vibration transmission through the device to be minimised.

The rods 24 are typically glued or welded to the rings 26, 27. Alternatively, the entire bearing support arrangement 23 may be manufactured from a single piece of material such that a single piece component is provided.

Alternatively the rods could be pre-stressed either in compression or in tension. Where this pre-stress is applied in tension, in order to increase the resistance of the arrangement to buckling, the flexible rods may be replaced by flexible threads or fibres. Applying the pre-stress in compression decreases the overall length of the arrangement.

Ideally the bearing support arrangement 23 is made from a material with high thermal conductivity such that an efficient heat path is provided for conducting heat away from the bearing 21. Example materials include beryllium copper, having a thermal conductivity value of 118 W/mK or phosphor bronze having a thermal conductivity value of 208 W/mK. These values are relatively high in relation to the corresponding typical value of a carbon steel, which would be in the region of 45 W/mK.

M04B1 08/KCM An alternative embodiment, illustrated in Figure 3 implements a bearing support arrangement 23 very similar to that of Figure 2, in conjunction with an elastomeric support ring 36. The support ring 36 is positioned between the bearing support arrangement and a modified housing component 35. It supplements the radial stiffness of the rods 24 and restricts the potential movement of the locator ring 26 which, in turn increases both stiffness and damping of the system.

Claims

6 M04B1 08/KCM Claims 1. A bearing arrangement for supporting a shaft

during rotation, the bearing arrangement comprising: a rolling bearing supporting a portion of the shaft, and a bearing support mechanism comprising a plurality of axially aligned members positioned about the rolling bearing, a first end of each axially aligned member being attached to an outer race of the rolling bearing, and a second end of each axially aligned member being attached to a non- rotating housing component.
2. A bearing arrangement according to Claim 1, wherein the axially aligned members are rods.
3. A bearing arrangement according to Claim 2, wherein the rods are 1 5 flexible.
4. A bearing arrangement according to any preceding claim, wherein the bearing support mechanism comprises a first locator ring, the first end of each axially aligned member being attached to the first locator ring.
5. A bearing arrangement according to Claim 4, wherein the first locator ring is attached to the outer race of the rolling bearing.
6. A bearing arrangement according to Claim 4 or Claim 5, wherein the axially aligned members are integral with the first locator ring.
7. A bearing arrangement according to any of Claims 4 to 6, wherein the bearing support mechanism comprises a second locator ring, the second end of each axially aligned member being attached to the second locator ring.
8. A bearing arrangement according to Claim 7, wherein the second locator ring is attached to the non-rotating housing component.

M04B1 08/KCM
9. A bearing arrangement according to Claim 7 or Claim 8, wherein the axially aligned members are integral with the second locator ring.
10. A bearing arrangement according to any preceding claim, wherein the bearing support mechanism is a single component
11. A bearing arrangement according to any preceding claim, wherein the bearing support mechanism is made from a thermally conductive material, such as beryllium copper or phosphor bronze.
12. A bearing arrangement according to any preceding claim, wherein the bearing support mechanism carries a support ring.
13. A bearing arrangement according to Claim 12, wherein the support ring is made from an elastomeric material.
14. A bearing arrangement according to Claim 12 or Claim 13, wherein the support ring is positioned about the bearing support mechanism.
15. A rotating machine having a bearing arrangement according to any preceding claim.
16. A vacuum pump comprising: a pumping arrangement; a shaft for driving the pumping arrangement; a motor for rotating the shaft; and a bearing arrangement according to any of Claims 1 to 14.